1. Field of the Invention
This invention relates to the field of sensors for detecting chemical and biological materials. More particularly, it pertains to the novel technique of fabricating integrated micromachined cantilever chemical and/or biological sensors.
2. Description of the Related Art
The need for detection of chemical and/or biological agents in a variety of applications is acute. The rapid detection of very small quantities of harmful molecules, DNA, viruses, etc. using cheap throw-away sensors is particularly important.
A number of methods have been developed which allow such detection. Microelectromechanical (MEMS) technology possesses a major role in this field since MEMS sensors can be batch-processed for low cost and are capable of handling and detecting very small quantities of unknown substances. Small amounts of materials, often in the range of pico or femto liters, can be handled and measured.
In the field of MEMS sensors, cantilevers micromachined from silicon or silicon nitride with sub-millimeter lengths, widths, and thicknesses less than 10 micrometers have been used in sensors for detection of chemical and/or biological agents.
For instance, the U.S. Pat. No. 5,372,930 to Colton discloses a microfabricated cantilever which was originally developed for atomic force microscopy but was later utilized in biological and chemical sensing. The proximal probe serving as a biological and chemical sensor measures the forces arising from molecular interaction between a chemically modified probe (attached to the microfabricated cantilever) and a chemically modified surface.
Two other chemical sensors based on microfabricated cantilevers were discussed in the U.S. Pat. No. 5,807,758 to Lee, et. al.
Lee, et. al. compares these sensors with surface acoustic wave (SAW) detectors which are known to use substrates with coatings that selectively bind to target molecules of interest. When the target species binds to the coating, the additional mass of the coating will change the resonant frequency of a substrate surface acoustic wave.
One sensor discussed by Lee, et. al. utilizes the thermally induced stress produced by reactions catalyzed on the metallic coating of the cantilever. Two limitations of this sensor are that it operates most effectively in a vacuum (requiring substantial instrumentation and making it unsuitable for most biological interactions that are desired) and it is limited in specificity by the reactivity of the metal coating.
Another sensor uses the change in the resonance frequency of the cantilever due to the mass of the chemical species, in a manner analogous to the change observed in sensors using the SAW frequency. Mercury was non-specifically detected in the demonstration of the sensor, although it is noted that chemically active surfaces may be used for the specific identification of analytes.
Noting the deficiencies of the sensors mentioned above, Lee et. al. proposes a new cantilever-based sensor. The target molecule to be determined using the sensor described by Lee et. al. may be in liquid phase (in solution) or, for some embodiments of the invention, in vapor phase. A sensor according to the Lee et. al. invention, monitors whether a target species has selectively bound to groups on the cantilever surface by monitoring the displacement of the cantilever, and hence the force acting on the cantilever.
Lee's sensor has major drawbacks and disadvantages that also characterize other sensors discussed below.
Previously, in all prior art describing cantilever beams of silicon or silicon nitride used for chemical or/and biological sensors, the cantilever beams have been fabricated on a wafer, diced, and then individually coated with the functionalization material.
Thus, if different materials were needed on different cantilevers for selectivity or temperature compensation, the cantilevers needed to be spaced far enough apart so that they could be manually dipped in separate reservoirs. This prevents the integration of closely packed cantilevers in a sensor and therefore increases the size and the manufacturing costs.
In addition according to the prior art, the individual sensor dies must be functionalized serially which also increases the time and cost of manufacturing. Finally, in many previous MEMS-based chemical or/and biological sensors, the detection of the cantilever motion was determined by optical laser-based techniques. See, for example, M. Maute, et. al., Detection of Volatile Compounds with Polymer Coated Cantilevers: Changes in Resonances of Thermal Noise, Transducers, 1999, pp. 636-639; B. Ilic, et. al., Mechanical Resonant Immunospecific Biological Detector, Phys. Lett., vol. 77, No. 3, pp. 450-452, July 2000; J. Fritz, et. al., Translating Biomolecular Recognition into Nanomechanics, Science, vol. 288, pp. 316-318, April 2000. In none of the prior art documents, were integrated control or sense electrodes incorporated for compact and low-cost sensors. Thus, these techniques yielded expensive, cumbersome and sluggish sensors.
In view of the foregoing, there is a need for a simple, inexpensive and accurate cantilever-based sensor for detection of chemical and biological materials. There is no known prior art which teaches a sensor satisfying these requirements yet a need to have such a sensor is acute.
By using the processing method of the present invention, on-chip electrostatic actuation and capacitive detection can be used for detecting resonance frequency or stress changes. These on-chip techniques provide for a sensor which is smaller, cheaper, more robust, and having improved performance not found in any other sensor device for the detection of chemical and/or biological materials.